Structure, electron denstiy and thermal motion of KCuF3

Thermal parameters from a refinement with newly collected data for KCuF3 [Mr = 159.64, tetragonal, I4/mcm, a = b = 5.8569(6), c = 7.8487(8) A, V = 269.24(6) A3, Z=4, Dx = 3.938 Mg m-3, λ(Mo Kα) = 0.71069 A, μ = 9.747 mm-1, F(000) = 300, T = 298 K, final R = 0.020, wR = 0.014 for 655 unique reflections] are consistent with those from a reanalysis of previously published data with a revised model including an isotropic extinction correction. There are two distinct types of F atoms and three different Cu-F bonds. The amplitude of the Cu-F2 stretching mode is largest for the long Cu-F2 bond. The single refined positional parameter differs significantly from previous determinations but the difference is shown to be due to bias in the redistribution of bonding electron density. Its value from refinement with full data gives a more satisfactory description of the net force at equilibrium geometry than that based on high-angle refinement. Effective atomic charges for Cu, K and F2, calculated by partitioning the difference density, are consistent with those in related structures but F1 is positively charged. This is explained by the strong depleting effect of exchange on the electron density, owing to structural compression along the c axis. The positive charge on F1 accounts for changes in K-F and F-F distances from the ideal perovskite values. There is excess electron density of 1.61 e A-3 located 0.45 A from F2, indicating strong dipole polarization along the Cu-F2-Cu bond. The shape of the depletion of electron density near Cu is broadly compatible with that expected for a cooperative Jahn-Teller distorted Cu system, but its radial dependence suggests depletion of paired spins along the shorter Cu-F bonds and of unpaired spins along the longest Cu-F bond. (orig.)